Frequency adjustment of piezoelectric resonators by ion etching has been known for decades. A summary is given in xe2x80x9cA Survey of Ion Beam Techniques for Piezoelectric Device Fabricationxe2x80x9d by R. N. Castellano et al, published in the xe2x80x9cProceeding of the 29th Annual Symposium on Frequency Controlxe2x80x9d, 1975, pp 128-134. However, inspite of this and other publications, the method has only recently been applied to the large-scale commercial frequency adjustment of electroded resonators.
In the adjustment process the electroded surface of the resonator is exposed to the beam of an ion gun in vacuum. The beam bombards and etches off a surface layer, thereby reducing the electrode mass and raising the resonator frequency toward a target value.
The process creates heat in the resonator that is proportional to the etch rate. Since the resonator frequency is affected by heat, it drifts as the resonator cools down after adjustment, thereby reducing the accuracy of the adjustment. The accuracy can be improved by lowering the etch rate, but this reduces the system throughput. Efficient and accurate large-scale processing therefore requires maximizing both the throughput and the accuracy of the adjustment system. This application describes such a system.
To our knowledge, the published reports of frequency adjustment by ion etching have used ion guns of the so-called xe2x80x9cKaufmanxe2x80x9d design. These guns include two heated filaments, one serving as a grid for controlling the ion beam, the other for neutralizing the ion beam. These filaments require relatively frequent maintenance.
The systems described in this application employ a so-called xe2x80x9cAnode Layerxe2x80x9d gun, which has no filaments and therefore requires little maintenance. However, it has the beam pattern of a closed, relatively narrow path rather than a contiguous area. The path can be circular or partially linear, but it must include curves for the path to be closed.
The invention relates to the frequency adjustment of piezoelectric resonators by ion etching in vacuum. It describes a novel system for maximizing both the throughput and accuracy of the adjustment. It is based on mounting the resonators on a tray in rows and columns and simultaneously exposing two rows at a time to an ion gun with a race-track-shaped beam pattern whose two straight tracks are spaced at a multiple of the inter-row distance d. The tray can be moved in steps of d such that each row is sequentially exposed to a xe2x80x9cpre-etchxe2x80x9d and xe2x80x9cfinal-etchxe2x80x9d stage, with time between the two exposures for the resonators to cool down after the xe2x80x9cpre-etchxe2x80x9d stage. The accuracy can be further enhanced by making the final-etch rate smaller than the pre-etch rate.